This invention addresses the problem of obtaining accurate positional, navigational, and timing (PNT) information in adverse physical environmental conditions (e.g., under jungle canopy or within urban canyons) as well as under adverse radio frequency (RF) environmental conditions (e.g., under jamming conditions or in the presence of spoofing signals). This invention presents a solution that is backwards-compatible with existing GPS technology.
Whether the adverse environment is natural or manmade, there are many scenarios under which existing GPS technology does not perform well. For example, under the cover of dense jungle foliage or from the inside of buildings, the GPS signal strength is often so attenuated that it becomes unusable. When users are deliberately jammed or spoofed by adversaries, a different set of issues may arise. While the GPS constellation is undergoing an upgrade which will modernize the GPS signal (e.g., the L1C signal), in the near term, the space vehicles (SVs) or GPS satellites 500 currently in service are fixed in their orbits. FIG. 1. It is not possible or feasible for the existing GPS satellites 500 to increase their power output, change their transmission frequency, or change their message structure.
Other approaches to addressing the adverse environment problem have considered specialized signal processing on the GPS receiver 401 to increase the processing gain in order to to allow the signal to be acquired and tracked at a lower signal level. However, the problem with this approach is that existing commercial GPS chips are widely deployed, have been heavily tested, and have extensive certification processes in place. These existing commercial GPS chips utilize the existing GPS signal parameters. Any approach utilizing unique processing requirements would necessitate costly bespoke GPS receiver design, development, and testing. Additionally, these solutions are typically more power-hungry and slower than conventional GPS receivers.
Another possible solution to the problem of obtaining accurate PNT in adverse environments is rebroadcasting the GPS signal within the current GPS band. The problem with this is that GPS receivers calculating their position based on the rebroadcast signal will end up calculating the position of the rebroadcasting station as opposed to their own position. In order for such a system to be practically usable, a separate method of obtaining range and bearing from the rebroadcast station is necessary. Such a solution however, may have adverse effects on other users trying to receive the actual GPS signals due to interference. Additionally, the testing and fielding of such a system presents a myriad of regulatory burdens, as special exemptions are required to transmit any kind of energy in the GPS RF band.
A system receiving GPS-like signals from pseudolites or other transmitters on a radio frequency different from the GPS band is another potential solution. However, these solutions require a separate stand-alone receiver which would only work when within range of the pseudolites. This would mean that the end user needs to carry an additional device with them. Such an alternative GPS would require a separate battery, screen, and antenna as well as utilize an entirely different set of electronics. Deployed en masse, such a solution would incur massive additional costs when compared with using existing GPS solutions.
Less complicated and less accurate methods are also possible, such as reliance on inertial navigation units (INUs) to maintain and update positional and navigational data in adverse environments, coupled with precision clocks (e.g., chip-scale atomic clocks) to maintain and update good timing. These devices however, are typically very delicate and not as accurate as the robust existing GPS technology. Such devices also require calibration prior to entering the adverse environment in the first instance, whereas the GPS is self-calibrating. A GPS system with a “cold start” (i.e., beginning with no information about where it is or what time it is) can have a highly accurate PNT solution in approximately 30 seconds.
Finally, the least accurate and most manually intensive methods are those such as dead reckoning or celestial navigation (e.g., reliance on maps and compasses or sextants). These approaches are typically used only as a last resort, and are not considered a feasible solution under most circumstances. Certainly, most of these types of alternatives are difficult from under heavy foliage cover or from within buildings.